Is there a diagnostic mode in the RCM?
Yes, the RCM has a special mode which you can use to troubleshoot your Sony PS-X75/65. If you have a Turntable that ran fine before its microcontroller failed, it should continue to function right after the RCM has been installed in the place of the original micro. However quite often you will be restoring an old PS-X75/65 that has more issues than just a failed micro. In such case the replacement of the micro may not be enough and you may need to perform some additional restoration work. The RCM's diagnostic mode can help you there.
How to activate the diagnostic mode?
Hold down the STOP button while powering up the PS-X75/75. You should hear a single beep of the buzzer installed on the RCM to confirm the activation of the diagnostic mode. Also in this mode the LED on the RCM will start flashing at the rate of 1 / sec
What exactly does the diagnostic mode do?
In the diagnostic mode all functions of your PS-X75/65 will work normally, but you will hear a short beep from the buzzer at any of the following events:
when you press or release any of the control buttons on the turntable
when the tonearm crosses any of the following locations along its path in this order:
the zero reference position (somewhere between the arm rest and the outer edge of the platter
the 30cm record stylus drop point (the record's lead in groove)
the 17cm record stylus drop point (the record's lead in groove)
the 30cm record arm lift-up point (the record's lead-out groove)
the 17cm record arm lift-up point (the record's lead-out groove)
What sort of problems can the diagnostic mode help with?
The diagnostic mode allows you to easily verify if the turntable's buttons function correctly or nor (e.g. due to aging). If you don't hear the beep when you press or release a button it means that the button action does not reach the RCM, hence making that particular function unavailable.
Also, if the beeps don't occur in the expected locations along the path of the moving tonearm that indicates problems in the A-B slit counting circuitry or in the zero reference position detection circuit.
What other problems to look out for?
The RCM heavily relies on the signals coming from the limit switches (marked 1 and 2 in the photo below). These switches are activated by the rotor of the arm's elevator drive (marked 3 in the photo).
Before initiating any arm's motion the RCM makes sure that the arm is raised and if it is not, it raises the arm first. The only way for the RCM to know the position of the arm's elevator is by reading the state of the limit switches (1 and 2). Any of the following problems will prevent the RCM from moving the arm, which means that no automatic actions like STOP, START, Auto-RETURN, or no manual control of the arm via the < > buttons will work. The frequently occuring problems are:
clogged bearing of the rotor (3). The residue of dust and old grease accumulating in the bearing over time will increase the friction in the bearing and as a result the rotor will not be able to rotate all the way to its extreme positions and thus won't activate the corresponding switch signalling to the RCM that the arm finished it downward or upward travel.
oxidized or dirty contacts in the switches (1, 2) will have the same result and the RCM will not be able to establish whether the arm is fully raised or fully lowered.
It is a fortunate fact that both the bearing of the rotor and the switches can be disassembled and cleaned fully restoring their original functions.
